It speaks to their incompetence really, because when I look at the list of subs I have subscribed to and interacted with, my Reddit history is very, very closely aligned with my interests, much more so than any of my other social media profiles. If they can't mine that data effectively then they don't deserve to make money.
The problem mentioned by parent comment is that it basically ends there: they struggle to cross-reference your Reddit persona (faithful ad it may be) with the bucketloads of signals advertisers get from other sites.
If you talk about buying a car on some random app, chances are that you'll then see ads on Facebook about used cars. Facebook gets to know what you do elsewhere, and offers advertisers extremely precise (and hence more valuable) targeting. That's what Reddit can't do.
They need a pixel/SDK to make that work. That should have been step 1 in relevancy. Seems like they could also gave used their sharing buttons but whoever is running their ads strategy may not be focused on the right things (i.e. Dr advertisers from adult and gaming to start with).
